This invention relates generally to processing of pulp for papermaking and more particularly to improvements in press vats for dewatering a papermaking pulp slurry.
Processing papermaking pulp requires exposure of the pulp to washing, various bleaching systems, flash drying, wet transport, alkali cellulose preparation, asphalt dispersion, high consistency fiber treatment, and other operations or combinations thereof. At various stages of the process, it is desirable to drastically increase the pulp consistency. This is usually accomplished by filtering, pressing, or combinations of the two operations.
One known pressing device consists of a cylinder formed from a perforated screen in which a constant pitch continuous flight tapered screw is driven to transport the pulp slurry. The low consistency pulp is usually gravity fed to the inlet of the press and is transported by the screw toward the outlet end of the press. As the pulp is conveyed along the screw in the ever decreasing screw chamber, it is also subject to the pressing action caused by the back pressure exerted by the discharge doors. In this process, it is possible to increase pulp consistency from a feed consistency of approximately three percent to a discharge consistency of approximately thirty percent.
Another effective device for dewatering pulp is the twin roll press, which has two closely spaced porous rolls between which the pulp is passed for dewatering. Typically, the pulp is fed under pressure, at a consistency of two to ten percent, to both rolls where it forms a mat and, by pressure filtration, its consistency is increased to approximately twelve to fifteen percent. Some such units have a so-called nip wedge below the rolls, which is rather closely contoured to the shape of the rolls and which serves to compact the pulp mat further as it approaches the nip between the rolls. While passing between the rolls, the pulp mat is severely compressed so that its consistency increases to as much as approximately fifty percent. It is then removed for further processing. During extreme compression some of the liquid in the pulp mat flows out of the nip forward but then falls back into the vat taking with it pulp fibers--if this does not occur the pulp mat can contact the nip wedge surface and adhere causing severe plugging.
Press units which are not equipped with the so-called nip wedge have a number of vat access doors, or manholes, in the vat bottom which provide a limited access to the underside of the rolls. These are required in order to clear occasional plugs which may occur if some of the dewatered pulp sheet sloughs off the rolls and accumulates to form a vat plug. If plugging is too severe, the vat access doors may not provide sufficient working room to clear the plug, in which case it may become necessary to disassemble the press. This is a very complex, costly, and time-consuming process.
One twin roll press provides improved access to the bottom of the rolls by use of a vat bottom door which can be lowered away from the vat bottom without disassembly. This improves access to the roll bottoms, but still requires that work be performed between the rolls and the vat bottom door which remains directly beneath the vat. Under these circumstances, it is still difficult to achieve access necessary to completely clear a severe plug.
The foregoing illustrates limitations known to exist in present pulp dewatering presses. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.